During the Baiu season, a stationary band-shaped precipitation system, termed 'Senjo-kousuitai' in Japanese, brings a long-lasting tremendous rainfall in the same place that often causes severe disasters. To understand the developing and maintaining mechanisms of the band-shaped precipitating systems, three aircraft observations using novel dropsondes developed by Nagoya University and Meisei Electric Co., Ltd. were performed in July 2025.
On July 14, 2025, a band-shaped precipitating system developed over the warm sea south of the main island of Japan. In total, 44 dropsondes were launched from an aircraft flying at an altitude of 14 km and obtained detailed vertical profiles throughout the troposphere across and along the band-shaped precipitating system extending from the south to the north. On the eastern side of the band-shaped precipitating system, a distinct low-level jet developed along the system traveling eastward. Above the 0oC-level, nearly-saturated moist layers extended widely to the 100-km east of the system. Below the nearly-saturated moist layers, an abundant water vapor was confined in the planetary boundary layers (PBL). The vertically high-resolution dropsonde profiles indicated inversion layers that 'capped' the moist PBL. Thus, without initiating convection, the abundant water vapor over the warm sea efficiently supplied to the band-shaped precipitation system. These distinct structures found by dropsondes were not represented by the JMA mesoscale analysis.
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